A voltage controlled oscillator (VCO) is simply an oscillator having a frequency output that is proportional to an applied voltage. While VCO's have many applications in a variety of electronic circuits, one particular application focused upon in the present disclosure is in communications systems in phase locked loops (PLL). A simple block diagram of a phase lock loop is as shown in FIG. 1. The basic elements of the system are a phase comparator, a loop filter, an amplifier and the voltage control oscillator. When the loop is in what is known as a locked condition on an incoming periodic signal, the VCO frequency is exactly equal to that of the incoming frequency. The phase detector produces a dc or low-frequency voltage which is proportional to the phase difference between the incoming signal and the VCO output signal. This phase sensitive signal is then passed through the loop filter and amplifier, where upon it is amplified to control the input of the VCO and the feedback loop. If, for example, the frequency of the incoming signal shifts slightly, the phase difference between the VCO signal and the incoming signal will begin to increase in time. This change in the control voltage on the VCO will effect a change to bring the VCO frequency back to the same value as the incoming frequency. Through this ongoing capturing process, the loop can maintain a lock on the input frequency changes, and the VCO input voltage is proportional to the frequency of the incoming signal. This behavior makes phase lock loop particularly useful in demodulation of FM signals, where the frequency of the incoming signals vary in time by definition and further contain the desired data or other information.
The range of input frequency over which a loop can maintain lock is known as the lock range. In an unlocked condition, the phase detector can be as simple as a multiplier-phase detector containing the sum and difference frequency components. The sum frequency component is generally filtered out by the loop filter, and the output of the low pass filter is then a sinusoid with a frequency that is equal to the difference between the VCO free running frequency and the incoming signal frequency. Application of this different frequency to the VCO input causes the VCO frequency itself to become a sinusoidal function of time. Because the VCO frequency is varying as a function of time, it will alternately move closer and farther away in frequency from that of the incoming signal frequency. The output of the phase detector is a near sinusoid with a frequency that is the difference between the VCO frequency and the input frequency. When the VCO frequency moves away from the incoming frequency, the sinusoid moves to a higher frequency. When the VCO frequency moves closer to the incoming frequency the sinusoid moves to a lower frequency. The result is that the frequency of the sinusoid different-frequency waveform is reduced when the incremental amplitude is negative and increased when the amplitude is positive. This causes the phase detector output to have an asymmetric waveform during capture. This asymmetric waveform introduces a dc component in the phase detector output that shifts the average VCO frequency towards the incoming signal frequency, so that the difference frequency gradually decreases. In this inherently non-linear process, the result is that the system becomes locked and the difference frequency becomes zero with a resultant dc voltage remaining at the loop filter output. This is known as the capture process and is a fundamental portion of a phase lock loop and the key function of the voltage controlled oscillator in a phase lock loop. Finally, the VCO is a critical element in PLL's as the VCO determines operating frequency range, FM distortion and center-frequency drift.
In conventional voltage controlled oscillators, discrete elements are generally used and mounted on an rf board. Thereafter, solder and wirebonds are used to effect the necessary interconnections between the elements, both passive and active, which are mounted on the rf board. This hybrid structure is generally relatively large in size as well as expensive. The cost of fabrication of the conventional hybrid VCO is becoming unacceptable for the wireless industry. To this end, the rf boards having the VCO with discrete elements are generally fabricated one at a time requiring high frequency interconnections as well as transformations. Additionally, impedance matching is often required and can be painstakingly tedious due to the various and sundry elements, for example solder paths and wire bonds.
Accordingly, what is needed is a voltage controlled oscillator having the required performance characteristics for high frequency applications at a substantially lower cost. The attendant disadvantages of discrete element VCO manufacture must be addressed and an integrated circuit VCO which is readily adaptable to large scale, wafer scale manufacture must be employed.